1. Field of the Invention
The present invention relates to an information recording and reproducing apparatus which uses a magneto-optical disk, and more particularly to a reproduced signal processing apparatus which converts an analog signal reproduced from stored information to a digital signal.
2. Related Background Art
A magneto-optical disk has been recently known as a large capacity and erasable and rewritable optical disk.
In such a magneto-optical disk, an orientation of magnetization is changed in accordance with information data by using a vertically magnetized film as a recording media in which the magnetization is oriented vertically to a disk plane so that binary information is recorded. A specific principle of recording and reproducing is described below.
The recording medium has a coercive force at room temperature so that the direction of magnetization is not changed. However, when a laser beam is irradiated to the recording medium, a temperature at the irradiated area suddenly rises, and when it reaches a Curie point, the coercive force decreases. Under this condition, if a weak magnetic field of the opposite polarity to the direction of the initialized magnetization of the recording medium is externally applied, the magnetization at that area is reversed. In this manner, data is written. The data is reproduced by irradiating a laser beam to the disk and detecting a change of polarized plane (Kerr effect) of a reflected light.
When data is to be recorded on such a magneto-optical disk, the data is normally recorded after the data has been converted to a modulated code which highly matches a characteristic of the recording and reproducing system. Either 4/15 modulation or 4/11 modulation has been proposed as the modulation system therefor. In the 4/15 modulation, an 8-bit data word is converted to a 15-bit channel word, and four positions of a 15-position pattern are set to "1". The "1" bits are assigned to two of odd-numbered positions and two of even-numbered positions, of the 15 positions.
Accordingly, when the channel word is to be demodulated to the data word, the channel word may be separated to odd-numbered bits and even-numbered bits. In the 4/11 modulation, an 8-bit data word is converted to an 11-bit channel word. Four positions of an 11-position bit pattern are set to "1". However, unlike the 4/15 modulation, 1 bits are not assigned to the odd-numbered and even-numbered bits.
The data recorded by the 4/15 or 4/11 modulation is usually reproduced by a differential detection system. FIG. 1 shows a block diagram of prior art differential detection means for the 4/15 modulated data.
In FIG. 1, a reproduced analog signal is applied at 1. The reproduced analog signal is sampled and held by a clock (not shown) for each data corresponding to one bit, by a sample and hold circuit 132, and it is converted to a digital signal by an A/D converter 7. The reproduced signal converted to the digital signal is supplied to a multiplexer 133 and then it is supplied to a comparator 134 or 135 depending on whether it is an even-numbered bit or an odd-numbered bit of a channel word.
The comparators 134 and 135 process in the same manner. Accordingly, the operation of only the comparator 134 is explained below. The comparator 134 sequentially stores the digitized reproduced signal into registers 141 and 142. Position numbers which indicate the bit positions of the reproduced signal, supplied from a counter (not shown) are also stored. When a third reproduced signal is supplied from the multiplexer 133 after the first two reproduced signals have been sequentially stored in the registers 141 and 142, the contents of the registers 141 and 142 are compared by a comparator 143 and a smaller one is outputted. It is compared with the third reproduced signal by a comparator 144. If the third input signal is larger, the content of one of the registers 141 and 142 which stores a smaller value is replaced by the input signal, and a position number which indicates a bit position of the input signal, supplied from a counter (not shown) is also stored.
If the output of the comparator 143 is larger, the third input is ignored. For fourth and subsequent reproduced signals, the comparison and storing are made in the same process. In this manner, when the 15 bits of one data word have been processed by the comparators 134 and 135, the contents of the registers 141, 142, 151 and 152 and the bit position data are transferred to a demodulator (not shown) where they are demodulated to the original data word.
The two largest signals each corresponding to the even-numbered address and the odd-numbered address in the data word of the sampled reproduced signal are sequentially extracted and the extracted reproduced signals are assigned to the "1" bits in the 15 bits. In the 4/11 modulation, the differential detection means does not process the signal by separating the even-numbered bits and the odd-numbered bit of the channel word, but the basic principle that the two largest signals of the sampled reproduced signal are sequentially extracted and the extracted reproduced signals are assigned to the "1" bits, is the same. However, in the above prior art differential detection system, a bit of the channel word which should be detected as "0" may be misdetected as "1" because of affect of waveform interference as shown in FIG. 2.
In FIG. 2, numeral 121 denotes a recorded data code (channel word), and numeral 122 denotes a reproduced analog waveform reproduced from the recorded code 121. When the reproduced analog waveform 122 is differentially detected, an order of the reproduced levels is that shown by 146. In this example, the third bit of bit No. 123 of the reproduced waveform 122 is detected as "1" while it should be "0". Accordingly, the detection code 147 is in error.